1. Field of the Invention
The present invention relates to a main element of a protector device and its fabrication method which returns itself to its non-conductive state in a very short time after conversion to its conductive state by a surge including thunder.
2. Related Background Art
A surge protector device including an arrester is very important device to protect various electronic devices from surge including thunder, where the term “thunder” means lightning due to an electrical phenomenon in the atmosphere. The surge protector device is a general name of devices which are used in order to protect other electronic devices from excess voltage, that is surge. An arrester is used to protect other electronic devices from thunder, that is extremely high voltage and large current. The arrester is one of the surge protector devices. The term of “protector device” is used here to indicate devices which are used in order to protect other electronic devices from excess voltage or excess current. However excess voltage is not limited to extremely high voltage such as thunder but includes low voltage if it is excess to a specified voltage.
A glass-tube type arrester has been used. It contains special gas between two electrodes in a glass tube. It is non-conductive unless surge is induced. When surge or thunder is induced, discharge starts and the gas between the electrodes changes to conductive. Current flows through the arrester and, it is lead to the earth. Discharge does not stop immediately after surge is removed. The arrester cannot protect other electronic devices from continuous current or next attack by surge or thunder. There are serious problems which a glass-tube and other type protector devices have which have been used. One of it is that a protector device must change from its resistive state to a conductive state in a very short time such as 0.03 μsec. when it is attacked by surge. Another problem is that a protector device should return from its conductive state to its resistive state when surge is removed.
In order to solve these problems in the prior art an arrester was proposed (Japanese Patent 118361, 1995 “Molybdenum arrester” by Seita Ohmori). It used a plural of molybdenum bars whose surface was oxidized. The arrester will be called here as a “molybdenum arrester”.
The molybdenum arrester leads current to the earth in a very short time when surge or thunder is induced. That is, it changes from non-conductive state to conductive state very quickly by breakdown of the oxide formed on the molybdenum bar. Moreover, it returns from conductive state to non-conductive state when surge or thunder is removed because molybdenum is oxidized quickly if it is in oxidizing atmosphere. The molybdenum arrester is very useful and economically efficient because it repeats change of the state automatically.
It is possible to use metals other than molybdenum in a protector device which functions with same principle as the molybdenum arrester. Tantalum, chromium and aluminum are included in such metals.
There is a serious problem in a protector device of the prior art which comes from the fact that the protector device uses a plurality of bars which have high resistive films on their surfaces. FIG. 1 shows schematically the protector (10) of the prior art which is called the molybdenum arrestor proposed by Ohmori (Japanese Patent 118361, 1995 Molybdenum arrester”).
The arrestor (10) includes two molybdenum bars (11) which have high resistive oxide films (12) on their surfaces and electrodes (13). The arrestor (10) uses breakdown phenomenon at the interface between the high resistive films (12). A breakdown voltage depends largely on microscopic structure of the interface. That is, as shown in FIG. 2, the high resistive films (12) on the two molybdenum bars contact point by point microscopically although they seem to contact line by line or surface by surface macroscopically. It is difficult to control the microscopic structure at the interface during fabrication process. Breakdown occurs at a point where largest electric field is applied by a surge. A breakdown voltage also depends on force induced to the interface. Therefore, it is impossible to design and fabricate the arrestor of the prior art with a precisely controlled breakdown voltage. The problem cannot be solved as far as a protector device uses breakdown phenomenon at the interface between two surfaces.
It is desirable, therefore, to provide a surge protector device which does not use breakdown phenomenon at the interface between two surfaces.